Anti-lock brake systems

ABSTRACT

An anti-lock vehicle brake system having a fluid pressure source for producing brake fluid under pressure at the wheel brake so as to apply braking pressure to the wheel in response to manual braking action by the operator. Also provided is a wheel movement sensing means for producing an output function in response to a related wheel rotational movement, and an anti-lock control valve actuatable in response to said output function and effective to divert brake fluid from the wheel brake so as to relieve braking pressure and thereby avoid wheel locking. A fail-safe arrangement is connected in the system for by-passing the anti-lock system in the event of malfunction of said anti-lock control valve so as to reapply full brake pressure to the wheel brake. The fail-safe arrangement includes a differential pressure member displaceable between a normal position and an operative position in response to a difference in fluid pressure acting on opposite ends thereof. Two different reference pressures act on different surface areas of the pressure member so that when the brake pressure falls to a critical low value the pressure member will be displaced from its normal position to its operative position in which a by-pass connection is established and fluid pressure is permitted to increase at the brake to restore braking pressure as a result of fluid being passed through the actuated control valve.

United States Patent 191 Skoyles ANTI-LOCK BRAKE SYSTEMS [75] Inventor: Derek Robert Skoyles, East Grinstead, England [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22] Filed: May 30, 1972 [21] Appl. No.: 257,961

Related U.S. Application Data [62] Division of Ser. No. 43,157, June 3, 1970, Pat. No.

[30] Foreign Application Priority Data June 27, 1969 Great Britain 32612/69 Aug. 6, 1969 Great Britain 39417/69 [52] U.S. Cl. 303/21 F, 188/181 A, 303/61 [51] Int. Cl B60t 8/02 [58] Field of Search... 3 03/21, 61-63,

[56] References Cited UNITED STATES PATENTS 3,232,676 2/1966 Cripe 303/21 F 3,403,945 10/1968 Dewar et a1. 303/21 F 3,414,336 12/1968 Atkin 303/21 F 3,449,019 6/1969 Walker 303/21 F 3,597,012 8/1971 Packer et a1. 303/21 F Primary E.raminerDuane A. Reger Assistant ExaminerD. C. Butler Amiz' Attorney, Agent, or Firm-Frank R. Trifari [5 7] ABSTRACT An anti-lock vehicle brake system having a fluid pressure source for producing brake fluid under pressure function and effective to divert brake fluid from the wheel brake so as to relieve braking pressure and thereby avoid wheel locking. A fail-safe arrangement is connected in the system for by-passing the anti-lock system in the event of malfunction of said anti-lock control valve so as to reapply full brake pressure to the wheel brake. The fail-safe arrangement includes a differential pressure member displaceable between a normal position and an operative position in response to a difference in fluid pressure acting on opposite ends thereof. Two different reference pressures act'on different surface areas of the pressure member so that when the brake pressure falls to a critical low value the pressure member will be displaced from its normal position to its operative position in which a by-pass connection is established and fluid pressure is permitted to increase at the brake to restore braking pressure as a result of fluid being passed through the actuated control valve.

6 Claims, 6 Drawing Figures 3,823,988 a [45-1 July 16, 1974 PATENTEU L 1 6 I974 SHEET 3 0F 5 jPREssuRE PIC-1.3.

PATENTED JUL 1 s 1914 SHEET 0F 5 107 106 114 108 $AIR 1 1 9e 97 1 Te 1 17 m 112 103 AIR F I G .4

PATENTEDJLUSIBYI 3.823388 m: 5 BF 5 TO VARIABLE RESTRICTOR T0 T0 MASTER RESERVOIR 27 CYLINDER UJ 2 :0) m8 BRAKING PRESSURE INCREASE DUE TO ORIGINAL FAIL-SAFE E CURRENT FAIL-SAFE RESPONSE VALUE RATE BRAKE PRESSURE MODIFIED FAIL- SAFE RATE DATUM Pa VALUE\ LOWEST X 1 4 VALUE 1 5 #TTIME CONTROL VALVE Y ACTUATED Fig.6

1 ANTI-LOCK BRAKE SYSTE This is a division, of application Ser. No. 43,157, filed June 3, 1970, now Pat. No. 3,708,213.

This invention relates to anti-lock brake systems for wheeled vehicles, that is, brake systems including means for improving braking performance of a vehicle by relieving braking pressure applied to a road wheel of the vehicle if the wheel tends to lock on a slippery surface following brake application and then increasing the braking pressure again without the need for any change in the actual braking action (by a person using the brake) causing the brake application. Such systems can be successful in reducing the risk of skidding due to wheel lock and in maintaining directional control during braking, and can also reduce braking distances.

The invention relates more particularly to a fluidpressure operated anti-lock vehicle-brake system, for use in conjunction with a vehicle wheel and associated wheel brake having, a fluid pressure source which in response to a braking action produces at the wheel brake fluid under pressure which causes the brake to apply braking pressure to the wheel, wheel movement sensing means for producing an output function in dependence on a particular criterion related to wheel rotational movement, and control valve means which is arranged for actuation in response to said output function and is effective upon actuation to reduce the pressure of said fluid at the wheel brake and thereby cause braking pressure to be relieved. A suitable criterion though not the only one is when the deceleration-of the wheel is in excess ofa predetermined value.

Normally, the period during which the control valve means remains actuated to relieve braking pressure is very short (i.e., only a few milliseconds), and the control valve means is released at the end of the .period to allow braking pressure to increase again. Thisnormal anti-lock operation to relieve braking pressure and then to allow it to increase again is repeated each time the wheel tends towards a locked condition that could result in skidding during a braking action.'However, if there isa malfunction preventing the release of the control valve means following its actuation, then-braking pressure would not be increased again by normal anti-lock operation and could continue to fall. Therefore, unless a failsafe arrangement of some form is provided, which is effective in the event of such a malfunction to over-ride the action of the control valve means and thereby allow brake application to be restored, a complete brake failure could result.

even though the abovemention'ed malfunction causes the braking pressure to become dangerously low. In other words, the fail-safe arrangement may remain unresponsive to the malfunction.

With a view to avoiding this danger the present invention provides an anti-lockvehicle brake system of the character referred to, wherein there is included a fail-safe arrangement comprising means which is con- .nected to be subjected to fluid pressure of value corresponding to braking pressure value and is responsive to the fluid pressure applied to it falling to a datum value, due to prolonged actuation of said control valve means, to over-ride the action of the latter to allow full brake application to be restored, together with meansfor automatically varying said datum value such that it is always slightly greater than the lowest -value of braking pressure which the system would otherwise produce with the control valve means actuated; In this way, reliance on an absolute value of fail-safe pressure is avoided. In a system in which locking value of braking pressure (i.e., the value that would lock a road wheel on a good surface) is approximately 1,500 psi, and the lowest value of braking pressure that the system can sustain with the control valve means actuated is approximately 50 p.s.i., a typical datum value of braking pressure may be approximately 1 percent or less of the locking value (i.e., 15 psi.) above the lowest value of braking pressure.

In carrying out the invention in respect of an antilock vehicle brake system of the character referred to wherein the control valve means is connected to or in I source of the system and the control valve means, said Such a fail-safe arrangement may be arranged to bepressure would have to be selected, below which the fail-safe arrangement becomes effective;

The present invention is based on the realization that if an absolute fail-safe" value of braking pressure is chosen, there is a danger that due to deterioration in the optimum performance of an anti-lock vehicle brake system (e.g., due to sticking" of anti-lock mechanisms thereof), this absolute value may never be-reached differential means being also arranged for displacement in the opposite direction to unseal said by-pass connection in response to the combined value of the pressure of said diverted fluid and fluid pressure from said source when the effective force exerted by said combined value on said differential means exceeds that exerted by the fluid pressure of braking value thereon. In a preferred embodiment of the invention, the fail-safe arrangement includes means for preventing fluid pressure from said source becoming effective for displacing said differential means until flow of fluid under pressure from said source, to exert fluid pressure of braking value at the brake, has ceased.

Whereas, the fail-safe arrangement according to the present invention as so far described is thought to provide satisfactory operation for a wide range of road surface conditions, it may not give an optimum performance for a very slippery (e. g., wet ice) surface. The reason for this is that the means therein which is responsive when braking pressure falls to said datum value takes no account of the time taken for the wheel to recover speed. It is known from an understanding of the essential requirements for anti-lock braking generally, that as soon as there is a tendency for a braked wheel to lock (due to excessive braking having regard to the prevailing road conditions), braking pressure is relieved until the wheel is rolling again and is then reapplied. In the case where there is good adhesion between the wheel and the road surface, the wheel accelerates again quickly. On the other hand,- the wheelv would take a much longer time to reach the free rolling condition again on a very slippery road surface. During such longer time, the braking pressure could fall to said datum value, even though there is no malfunction of the control valve means, in which case the fail-safe arrangement would become responsive spuriously to reapply braking pressure. The result of this is that the braking pressure could be re-applied suddenly to a wheel which is attempting toaccelerate from a lower speed on a slippery surface.

In 'orderto prevent the possibility of such aspurious response, the present invention further provides in conjunction with the fail-safe arrangement, restrictor means for causing braking pressure to which the failsafe arrangement is'responsive to fall at a slower rate than the rate at which braking pressure falls at the brake to'relieve braking pressure when the control valve means of the system is actuated.

The slower rate of fall of braking pressure to which the fail-safe arrangement is responsive would be so chosen that any deterioration of fail-safe performance for a good road surface would be minimized. There might be some deterioration because, as aforesaid, on agood road surface a freed wheel will become free roll ing again quickly, which means that actuation of the control valve means is for a short period only. Therefore, for a good road surface, a period of time constituting a prolonged actuation of the control valve means could be very slightly shorter than the period of time required for the slower rate of fall of braking pressure needed to give optimum fail-safe performance for a very slippery road surface. However, it is envisaged that an overall improvement in fail-safe performance would be achieved, since this instance of failure would be very rare.

In the case, as aforesaid, of a system in which the control valve means is connected to or in a pressure line for supplying fluid under pressure to the brake and the fail-safe arrangement comprises differential means, said restrictor means can comprises one-way valve means in a pressure line to which said differential means is connected to receive fluid pressure of braking value for displacement in said one direction, together with a restrictor element connected in a fluid connection in parallelwith said one-way valve means, the latter being so arranged as to permit fluid pressure of braking value to pass to said differential means, but to prevent such fluid pressure being relieved fromthe differential means, the relieving of this fluid pressure being through the restrictor element at a rate determined .thereby. The restrictor means may be, equivalent]y,-a leaky one-way valve connected in said fluid connection.

In order that the invention may be more fully understood, reference will now be made by way of example to the drawings in which FIGS. 1 to 4 show diagrammatically respective different forms of anti-lock vehicle brake system of the character referred to each including a fail-safe arrangement in accordance with the U.S. Pat. No. 3,724,914 issued Apr. 3, 1973. This U.S. system,

FlG. 5 shows diagrammatically'the fail-safe arrangement of the anti-lock brake system shown in FIG. 1;

and

FIG. 6 shows explanatory braking pressure/time curves.

Referring to the drawings, in FIG. 1 a fail-safe arrangement l is provided in a particular form of antilock vehicle brake system which is described in greater detail than herein in co-pending U.S. Pat. Application ,Ser. No. 881,460, filed Oct. 2, 1969. This system is a master cylinder type hydraulic system comprising a master cylinder 2 having a piston 3 which is actuable by a brake pedal 4. The system further comprises an anti-lock control valve 5, a wheel movement sensor 6, an electrical control arrangement 7, a variable restrictor element 8 and a scavenging pump 9. A vehicle brake 10 for a wheel 11 is controlled by this system.

In operation of the system of this FIG. 1 for normal brake application fluid in pressure lines 12 to 16 is pressurised by the master cylinder 2 to an extent determined by a drivers braking action and this produces a corresponding braking pressure in the brake 10. If the drivers braking action is likely to cause skidding due to the wheel locking, this is detected by the wheel movement sensor 6 and electrical control arrangement 7, and the control valve 5 is actuated. Details of the manner of operation and composition'of the elements 5, 6 and 7 are not thought to be necessary for an understanding of the present invention, but an example is given in the aforementioned U.S. Letters Patent.

As a result of the actuation of the control valve '5, some of the fluid under pressure in the pressure lines Band 16 is diverted into a pressure line 17, thereby relieving braking pressure. This diverted fluid enters a reservoir 18 of the variable restrictor element 8 where it is applied behind a spring-loaded plunger 19 and displaces this plunger to the right (as seen in the drawing), so that a restrictor 20 on the plunger 19 enters the pressure line 14, thereby restricting flow of fluid through this line to the line 15. Braking pressure is now reduced to an extent determined by the extent of displacement of the piston 19 to the right and thus to the volume of displaced fluid. For normal anti-lock operation the control valve 5 remains actuated for only the short time required to allow the braking pressure to fall suffrciently low for the wheel to re-accelerate. Once the control valve-5 is released, the pump 9, which is returning fluid from the reservoir 18 of the variable restrictor element 8 to the pressure line 14 via pressure line 21, gradually removes sufficient fluid from the reservoir 18 for the plunger 19 to return under its spring-loading towards its normal position, toincreasethe rate of braking pressure build-up, As indicated by the broken line 22, the pump 9 can be driven by the wheelll.

From the foregoing description it will be appreciated that if the control valve 5 remains actuated, due to a malfunction in the system, fluid needed in lines 15 and 16 to produce braking pressure would continue to be diverted through the control valve 5, with the result that, with the system as so far described, braking pressure would fall to a dangerously low value. The lowest value to which the braking pressure can fall is, in fact, the value of the reservoir pressure, that is, the pressure of fluid in the reservoir 18; The value of the reservoir pressure will therefore be the lowest value of braking pressure which the anti-lock brake system can produce if the control valve 5 remains actuated due to a malfunction. ln order to prevent the braking pressure falling to this dangerously low value, the fail-safe arrange- 'ment'l is provided. The function of this fail-safe arrangement is to determine whenever the braking pressure has fallen nearly to the reservoir pressure and to over-ride the normal anti-lock operation of the system to allow normal brake application arising from the drivers braking action to be restored.

The fail-safe arrangement 1 comprises a chamber 23 in which a piston member 24 is slidably accommodated. An O-ring 25 provides seal between the sides of the chamber 23 and the piston member 24. A pressure line 26 introduces fluidof braking pressure value into one end of the chamber 23 and a pressure line 27 introduces fluid of reservoir pressure value into the other end of the chamber 23. The fluid at braking pressure value introduced into the chamber 23urges the piston member 24 into the position shown in which an O-ring 28 on the member seals off a central passage 29 from the chamber'23. This O-ring 28, with the piston member 24 in this position, also seals off a central surface area A1 of the piston member 24 from the fluid at reservoir pressure value, so that this fluid exerts a force over a lesser-surface area A2 than the fluidof braking the anti-lock control valve 5 is actuated is determined pressure value which exerts a force over thewhole surface area A3 at the other side of the piston member 24.

For normal anti-lock operation of the system, the braking pressure which issignificantly greater than the reservoir pressure, and which also acts overa larger surface area on the piston member 24, maintains the piston member 24 in the position shown, so that the fail-safe arrangement lremains inoperable. However, in the event that the control valve 5 remains actuated for a prolonged period, braking pressure will fall as aforesaid and its value will tend towards that of the reservoir pressure. Fluid at master cylinder pressure is introduced through the passage 29 and initially is effective only at the small central surface area A1 of the piston member 24. When the force exerted by the braking pressure becomes less than the total force exerted by the reservoir pressure and the master cylinder pressure, the piston member 24 is displaced from its normal position and fluid at master cylinderpressurelocally increases the pressure to continue the displacement, this increase of pressure being effective over the total surface area (Al +A2) of the piston member 24. Fluid at master cylinder pressure enters the pressure line 27 and passes through this line into pressure line 17 and the reservoir 18, through the opened control valve 5 and to the brake 10 to allow full brake application due to a drivers braking action to be restored.

In order to prevent the fluid at master cylinder pressure becoming effective at the surface area A1 of the piston member 24 during normal brake operation, a slide member 30 is provided in the passage 29. When a braking action is initiated, fluid at master cylinder pressure flows from the master cylinder 2 through pressure line 12 to pressure line 13 andthis flow of fluid displaces the member 30 so that a restrictor 31 carried by the-member 30 is positioned in a neck portion 32 of the passage 29. This restrictor 31 considerably reduces the fluid at master cylinder pressure which can be applied against the surface area A1 of the piston member 24 during this flow of fluid. Fluid at braking pressure is able to displace piston member 24 to the right, thereby ensuring that the sea] at member 28 is complete. When the flow of fluid ceases, the member 30 is left floating and the master cylinder pressure is applied only against the surface area A1 of the piston member 24.

by the parameters of the system, which could be too short in certain circumstances and thus result in the shortcoming referred to previously.

' The rate at which braking pressure in chamber 23 of the fail-safe arrangement 1 falls consequent upon actuation of the anti-lock control valve of the system can be made longer by including in the pressure line 26 between the variable restrictor element 8 of the system and the chamber 23, as shown in FIG. 5, a restrictor means comprising a one-way valve 200 and a restrictor 201 connected in parallel therewith. Each time the braking pressure at the variable restrictor side of valve 200 exceeds the pressure in chamber 23, this valve opens, so that the new value of braking pressure is transferred to the chamber 23. If the braking pressure at the variable restrictor side of valve 200 suddenly falls (due to the anti-lock control valve of the system being actuated), then the pressure in chamber 23 will not fall at the same rate, as in the original system, but at a slower rate which is determined to a material extent by the restrictor 201 which by-passes the valve 200 to allow restricted flow of fluid under pressure from the chamber 23.

As shown in FIG. 6, in the curves of braking pressure P against time, the datum value of braking pressure at which the fail-safe arrangement is responsive to cause braking pressure to be re-applied, following the relieving of braking pressure consequent upon actuation of the anti-lock control valve of the system, is reached at a relatively short time tx in the original system and after a much longer time ty with the addition of the restrictor means to the original system.

Turning now to FIG. 2 of which shows diagrammatically a fail-safe arrangement according to the invention in an anti-lock vehicle brake system which includes a vacuum servo amplifier foreffecting anti-lock operation when an anti-lock control valve of the system is actuated. This FIG. 2 shows a vehicle 'wheel 33, a wheel brake 34, a wheel movement sensor 35, an electrical control arrangement 36, an anti-lock control valve 37, and a master cylinder 38 having a piston 39 which is actuable by a brake pedal 40.

In operation of the system of FIG. 2. fornormal brake application, fluid in pressure lines 41 and 42 is pressurized by the master cylinder 38 to an extent determined by a drivers braking action and this produces a corre sponding braking pressure in the brake 34. If the drivers braking action islikely to cause skidding, this is detected by the wheel movement sensor 35 and electrical control arrangement 36, and the control valve 37 is actuated. As in the system of FIG. 1, the elements 35, 36 and 37 may be as described in the aforementioned U.S. Letters Patent.

The vacuum servo amplifier of the system comprise an enclosure 43 which houses a diaphragm 44 that effectively divides the enclosure into two portions 45 and 46. The diaphragm 44 carries a piston 47 which extends through the wall of the enclosure portion 45 in a sealing and sliding manner and into a bore 48. An 0- ring 49 provided on the piston 47 provides a sealing engagement between the piston 47 and the wall of the bore 48. The end of the piston 47 carries a valve 50 which is adapted to seal an orifice 51. There is provided in the lower portion 46 of the enclosure 43 a spring 52 which engages the diaphragm 44 to urge it upwardly (as seen in the drawing), so that the valve 50 is displaced from its seating in orifice 51, as shown. In this position of the valve 50,'fluid pressure from the master cylinder 38 can pass between pressure lines 41 and 42v to actuate the brake 34, as aforesaid.

45. The effect of this air at atmospheric pressure in the enclosure portion 45 is to displace the diaphragm 44 against the spring 52 so that the piston 47 carried on the diaphragm 44 is displaced in a direction tending to seat. the valve 50 in the orifice 51 and thereby restrict braking pressure from the master cylinder 38 to the brake 34.'There is also an immediate relieving of braking pressure because displacement of the piston 47 increases the available volume of the bore 48 to the fluid in pressure line 42 and the bore 48.

If, due to a malfunction, the control valve 37 remains actuated for a prolonged period, then air at atmospheric pressure continues to flow into the enclosure portion 45 with the result that the valve 50 continues its displacemnt towards the orifice 51 so that braking pressure continues to be relieved and could fall to a dangerously low value. The aperture 54 in the diaphragm 44 is of insuff cient size to equalize the pressures at opposite sides of the diaphragm 44, to prevent the fall in braking pressure.

The fail-safe arrangement provided in this system to prevent the braking pressure falling too low comprises an enclosure 56 which is divided into two portions 57 and 58 by means of a diaphragm 59 housed therein. A connecting'line 60 extends between the enclosure portion 45 of the vacuum servo enclosure 43 and the enclosure portion 57 of the fail-safe enclosure 56. A piston 61 connected to the -diaphragm,59 extends into the other enclosure portion 58 of the fail-safe enclosure 56, which latter enclosure portion has an opening 62 through which it is exhausted to air at atmospheric pressure. A connecting line 63 extends between the enclosure portion 46 of the vacuum servo enclosure 43 and the fail-safe enclosure portion 58, but this line 63 is normally sealed from this enclosure portion by means of a sealing member 64 carried on the piston 61.

When the control valve 37 is actuated, air at atmospheric pressure from the enclosure portion 45 enters the enclosure portion 57 of the fail-safe enclosure 56 via the connecting line 60 to displace the diaphragm 59 therein to the right (as seen in the drawing). As the pressure of this air increases, as the vacuum in the enclosure portion 45 is overcome, the force applied by it to the diaphragm 59 increases until it overcomes the force applied by the pressure of air at atmospheric pressure to the other side of the diaphragm 59. Due to the piston 61 connectedto the diaphragm 59, the air at atmospheric pressure acts over a lesser surface area of the diaphragm than the air pressure from the enclosing portion 45, so that the latter air pressure will eventually displace the diaphragm 59 to the right before atmospheric pressureis reached. 1n so doing, the sealing member 64 carried on the piston 61 is moved to. connect the line 63 with the enclosure portion 58 with the result that air at atmospheric pressure in this enclosure portion extends through this line '63 into the enclosure portion 46 of the vacuum servo enclosure 43 and is applied against the diaphragm 44in opposition to the air applied to the other side of this diaphragm in the enclosure portion 45. Accordingly, these air pressures equalize so that the force applied by the spring 52 can become effective to-displace the diaphragm 44 and so cause'the valve 50 to unseat from the orifice 51 and thereby allow full master cylinder pressure to be reapplied to the brake 34. V Turning now to F IG. 3 which shows diagrammatically a fail-safe arrangement according to the invention in an anti-lock vehicle brake system which includes an air pressure servo amplifier for effectign anti-lock operation when an anti-lock control valve of the system is actuated. This FIG. 3 shows a vehicle wheel 65, a wheel brake 66, a wheel movement sensor 67, an electrical control arrangement 68, an anti-lock control valve 69, and a master cylinder having a piston 71 which is acutable by a brake pedal 72.

In operation of the system of FIG. 3 for normal brake application, fluid in pressure lines 73 and 74 is pressurized by the master cylinder 70 to an extent determined by a drivers braking action and this produces a corresponding braking pressure in the brake 66. 1f the drivers braking action is likely to cause skidding due to the wheel locking, this is detected by the wheel movement sensor 67 and electrical control arrangement 68, and the control valve 69 is actuated. As in the system of FIG. 1, the elements 67, 68 and 69 may be as described in the aforementioned U.S. Letters Patent.

The air pressure servo amplifier of the system comprises an enclosure 75 which houses a diaphragm 76 that effectivelydivides the enclosure into two portions 77 and 78. The diaphragm .76 carries a piston 79 which extendsthrough the wall of the enclosure portion 77 in a sealing and sliding manner and into a bore 80. An O ring 81 provided on the piston 79 provides a sealing engagement between the piston 79 and the wall of the bore 80. The end of the piston 79 carries a valve 82 which is adapted to seal an orifice 83. Normally, full air pressureexists in the enclosure portion 78 to urge the diaphragm 76 upwardly (as seen in the drawing), so that the valve 82 is displaced from its seating in orifice 83 as shown. In this position of the valve 82, fluid pressure from the master cylinder 70 can pass between pressure lines 73 and 74 to actuate the brake 66, as aforesaid. I

The air pressure in the enclosure portion 78 is provided by an air pressure source 84 via the anti-lock control valve 69 anda fail-safe arrangement 85. When the control valve 69 is actuated, to effect an anti-lock operation, it connects air at atmospheric pressure to the enclosure portion 78, instead of air pressure from the source 84. The effect of this is that the air pressure in the enclosure portion 78 starts to exhaust to atmosphere, so that the diaphragm returns (downwardly) towards a lower position. As a consequence, the piston 79 carried on the diaphragm 76 is displaced in a direction tending to seat the valve 82 in the orificet83 and thereby relieves braking pressure as applied from the master cylinder 70 tQ'thfi brake 66. There is also an immediate relieving of braking pressure becausedisplacement of the piston 79 increases the available volume of the bore 80 to the fluid in pressure line 74 and the bore 80.

If, due to a malfunction, the control valve 69 remains actuated for a prolonged period, then the enclosure portion 78 will continue to exhaust towards atmosphere with the result that the valve 82 continues itsv displacement towards the orifice 83 so that braking pressure continues to be relieved and could fall to a dangerously low pressure value. The fail-safe arrangement 85 provided in this system to prevent the braking pressure falling too low for too long comprises two enclosure portions 86 and'87 between which is provided a diaphragm 88. The enclosure portion 86 is normally connected between the control valve 69 and the enclosure portion 78 of the air pressure servo 75, and the enclosure portion 87 is connected via a connecting line 89 to the air pressure source 84. This line 89 is normally sealed from the enclosure portion 87 by a sealing member 90, carried on the diaphragm 88, due to the latter being displaced to the left (as seen in the drawing) by the air pressure in the enclosure portion 86. The enclosure portion 87 also has a small opening-91 through which it is exhausted to air at atmospheric pressure.

When the control valve 69 is actuated, the enclousre portion 86 is exhausted towards atmospheric pressure through the control valve 69. Thus the force acting on the diaphragm 88 to urge it to the left is reducing. Air at atmospheric pressure is already applying a force over a major portion of the surface area of the diaphragm 88, to urge the latter to the right, and a further force produced by the air pressure in line 89 is also acting in the same direction over a minor portion'of the surface area of the diaphragm, i.e.,- against the sealing member 90. These combined forces become effective to urge the diaphragm 88 to the right when the air pressure in the enclosure portion 86 is sufficiently reduced. Once the sealing member 90 disengages from the connecting line 89, the force of the'air pressure from the source 84 acts against the whole surface area of the diaphragm which is thus displaced rapidly to the right until another sealing member 92 carried on it seals the enclosure portion 86 from the actuated control valve 69. The air pressure from the source 84 passes from the enclosure portion 87 to the enclosure portion 86 via a one-way flap valve 93 and thence to the enclosure portion 78 of the air pressure servo 75 to displace the diaphragm 76 (upwardly) and so cause the valve 82 to unseat from the orifice 83 and thereby allow full master cylinder pressure to be re-applied to the brake 66. The line 91 is not of sufficient diameter for the air pressure to dissipate through it to an extent preventing the operation just described taking place.

FIG. 4 shows diagrammatically a fail-safe arrangement according to the invention in an anti-lock vehicle brake system which is an air pressure system for operating a wheel brake 94 associated with a vehicle wheel 95. This system comprises an air pressure source 96, an air pressure reservoir 97, a modulator 98 which is controlled by a brake foot pedal 99, an anti-lock control braking action, and this allows a corresponding extent of air pressure to pass from the reservoir 97, through the modulator 98 and the anti-lock control valve 100 to the brake 94 to actuate the latter to effect braking. If the drivers braking action is likely to causevskidding due to the wheel locking, this is detected by the wheel movement sensor 102 and electrical control arrangement 101, and the control valve100 is actuated. As in FIG. 1, the elements100, 101 and 102 may be as described in the aforementioned U.S. Letters Patent.

When the control valve 100 is actuated, to effect an anti-lock operation, it connects air at atmospheric pressure to a pressure line 104, instead of air pressure from the modulator 98, the effect of this being that air pressure in the brake 94 is reduced rapidly to'cause an immediate relieving of braking pressure. If due to a malfunction, the control valve 100 remains actuated for a prolonged period, then air pressure. in the brake 94 will continue to fall so that braking pressure continues to be relieved and could fall to a dangerously low pressure value. The fail safe arrangement 103 provided in this system to prevent the braking pressure falling too low for too long comprises two enclosure protions and 106 between which is provided a diaphragm 107. The enclosure portion 106 is connectedto the pressure line 104 and theenclosure portion 105 is connected to the output side of the modulator 98 via a pressure line 108. This line 108 is normally sealed from the enclosure p0- rition 105 by a sealing member 110, carried on the diaphragm 107, due to the latter being displaced to the left (as seen in the drawing) 'by the air pressure (from line 104) in the enclosure portion 106. The enclosure portion 105 also has a small opening 111 through which it is exhausted to air at atmospheric pressure.

When the control valve 100 is actuated, the air pressure. in the enclosure portion 106 falls towards atmospheric pressure due to its connection with line 104 in which air pressure is reducing, as aforesaid. Thus the force acting on the diaphragm 107-to urge it to the left is reducing. Air at atmospheric pressure is already applying a force over a major portion of the surface area of the diaphragm 107, to urge the latter to theright, and a further force produced by the air pressure in line 108 is also acting in the same direction over aminor portion of the surface area of the diaphragm 107, i.e., against the sealing member 110. These combined forces become effective to urge the diaphragm 107 to the right when the air pressure in the enclosure portion 106 is sufficiently reduced. Once the sealing member 110 disengages from the air pressure line 108, the force of the air pressure from the modulator 98 acts against the whole surface area of the diaphragm 107 which is thus displaced rapidly to the right and air pressure from the modulator 98 passes from the enclosure portion 105 to the enclosure portion 106 via a one-way flap valve 112 and thence via pressure line 104 to the brake 94 so that full modulator air pressure is re-applied to the brake. The line 111 is not of sufficient diameter for the'air pressure. from the modulator 98 to dissipate through it to an extent preventing the operation just described taking place. Also, in order to prevent the reapplied air pressure in line 104 from exhausting to atmosphere through the actuated control valve 100, a line 113 connected. to the control valve 100 through which the exhausting takes place is sealed by a second sealing member 114 when the diaphragm 107 is displaced to the right. This sealing member 114 iscarried at the end of a stem 115 which is secured to the'diathrough the wall of the' enclosure portion 106 and through the wall of the enlarged portion 116 of a line 113 in which the sealing member 114 is located. A spring 117 normally urges the diaphragm 107 to the left (so that air pressure can be exhausted to atmosphere past the sealing member 114 duringnormal anti-lock operation. However, once the diaphragm 107 is displaced to the right, due to a malfunction, the force exerted by the spring 117 having been overcome, air pressure in line 113 holds the sealing member 114 in its sealing position to prevent air in line 113 from exhausting to atmosphere.

' In each of the embodiments just described, recovery of the malfunction will allow normal anti-lock operation to take place, if necessary, in response to further braking action.

Although the invention has been exemplified in F165. 1 to 3 as applied to brake systems in which brake application is effected by directly acting master cylinder pressure, it is to be appreciated that the invention is equally applicable to servo-assisted master cylinder type systems.

An anti-lock vehicle brake system of the character referred to including a fail-safe arrangement according to the invention may be provided in respect of each road wheel of a vehicle, or two such systems may be provided for each of two front wheels of a vehicle with a third system provided jointly for the two rear wheels of the vehicle. The several systems in either case would share a common fluid pressure source.

I claim: a

1. An anti-lock vehicle brake system, comprising:

a wheel brake; a source ofbrake fluid pressure;

a brake connection between said brake source, and said brake;

means forapplying pressure to said brake from said source for operating said brake;

an anti-lock control valve for relieving said brake pressure when actuated;

means connected to said control valve for detecting wheel locking so as to actuate said control valve;

pressure fluid r'estrictor means disposed in said brake connection between said brake and said brake fluid pressure source for closing said brake connection and relieving said brake pressure upon actuation of said anti-lock valve; I

a vacuum servo amplifier supporting said restrictor means and comprising first and second chambers having a common movable wall member, and further containing biasing means for maintaining said restrictor means in a fluid flow position when said anti-lock control valve is unactuated, a vacuum source connectedto said second chamber of said servo amplifier, and an atmospheric pressure source connected to said first chamber when said anti-lock controlvalve is actuated, said pressure response tov prolonged actuation of said anti-lock conanti-lock control valve is actuated, whereby the restrictor means is moved to a fluid restricting position causing the brake pressure to decrease; and fail-safe arrangement for restoring brake pressure to said brake of said system in the event that said anti-lock control valve is subjected to prolonged actuation, said failsafe arrangement comprising an enclosure, a differential pressure member dividing said enclosure into first and second enclosure portions, a first connection connecting said first enclosure portion to one of said chambers of said servo amplifier, a nromally closed second connection connected between said second enclosure portion and the other chamber, means connected to said second enclosure for venting said second enclosure to atmosphere, said pressure differential member having a larger surface area on the side of said first enclosure portion than on the side of said second enclosure portion, valve means carried by said pressure differential member for opening said second connectionwhen the pressure in said first enclosure portion reaches a predetermined value so as to displace said pressure differential member, said supply of vacuum to said second chamber of said servo'amplifier being inhibited upon opening of said second connection to thereby actuate said biasing means to move said restrictor to said fluid flow positon and restore said brake pressure.

2. The anti-lock system of claim 1, wherein said servo amplifier further comprises means for increasing and decreasing the available volume for the fluid in said connection, said volume increasing in conjunction with restrictive movement of said restrictor means, and decreasing in'conjunction with movement of said restrictor means to said fluid flow position.

3. The anti-lock system of claim 2, wherein said volume means is a movable piston and seal arrangement for varying the connectionbore size.

4. The anti-lock'system of claim 1, wherein said valve member is carried bya movable piston connected to said pressure differential member and displaceable in trol valve, said second connection being'connected bea tween said vacuum source and atmosphere, the dissources combining to move said will member placement of said piston resulting in freeing a path in said connection for the vacuum source to vent to the atmosphere, whereby said vacuumsource is inhibited from supplying a-vacuum 'to said servo amplifier.

5. The anti-lock system of claim 4, wherein said piston is subjected to atmospheric pressure on differential surfaces thereof in response to anti-lock control valve actuation, causing it to be displaced and vent said vacuum source.

6. The anti-lock system of claim 1, wherein said vacuum servo amplifier comprises a dichotomous chamber separated into a vacuum section and an atmospheric section by. a diaphragm, said sections communicating with each other through a small aperture in said diaphragm. 

1. An anti-lock vehicle brake system, comprising: a wheel brake; a source of brake fluid pressure; a brake connection between said brake pressure source, and said brake; means for applying pressure to said brake from said source for operating said brake; an anti-lock control valve for relieving said brake pressure when actuated; means connected to said control valve for detecting wheel locking so as to actuate said control valve; fluid restrictor means disposed in said brake connection between said brake and said brake fluid pressure source for closing said brake connection and relieving said brake pressure upon actuation of said anti-lock valve; a vacuum servo amplifier supporting said restrictor means and comprising first and second chambers having a common movable wall member, and further containing biasing means for maintaining said restrictor means in a fluid flow position when said anti-lock control valve is unactuated, a vacuum source connected to said second chamber of said servo amplifier, and an atmospheric pressure source connected to said first chamber when said anti-lock control valve is actuated, said pressure sources combining to move said wall member thereby to overcome said biasing means when the anti-lock control valve is actuated, whereby the restrictor means is moved to a fluid restricting position causing the brake pressure to decrease; and a fail-safe arrangement for restoring brake pressure to said brake of said system in the event that said anti-lock control valve is subjected to prolonged actuation, said failsafe arrangement comprising an enclosure, a differential pressure member dividing said enclosure into first and second enclosure portions, a first connection connecting said first enclosure portion to one of said chambers of said servo amplifier, a nromally closed second connection connected between said second enclosure portion and the other chamber, means connected to said second enclosure for venting said second enclosure to atmosphere, said pressure differential member having a larger surface area on the side of said first enclosure portion than on the side of said second enclosure portion, valve means carried by said pressure differential member for opening said second connection when the pressure in said first enclosure portion reaches a predetermined value so as to displace said pressure differential member, said supply of vacuum to said second chamber of said servo amplifier being inhibited upon opening of said second connection to thereby actuate said biasing means to move said restrictor to said fluid flow positon and restore said brake pressure.
 2. The anti-lock system of claim 1, wherein said servo amplifier further comprises means for increasing and decreasing the available volume for the fluid in said connection, said volume increasing in conjunction with restrictive movement of said restrictor means, and decreasing in conjunction with movement of said restrictor means to said fluid flow position.
 3. The anti-lock system of claim 2, wherein said volume means is a movable piston and seal arrangement for varying the connection bore size.
 4. The anti-lock system of claim 1, wherein said valve member is carried by a movable piston connected to said pressure differential member and displaceable in response to prolonged actuation of said anti-lock control valve, said second connection being connected between said vacuum source and atmosphere, the displacement of said piston resulting in freeing a path in said connection for the vacuum source to vent to the atmosphere, whereby said vacuum source is inhibited from supplying a vacuum to said servo amplifier.
 5. The anti-lock system of claim 4, wherein said piston is subjected to atmospheric pressure on differential surfaces thereof in response to anti-lock control valve actuation, causing it to be displaced and vent said vacuum source.
 6. The anti-lock system of claim 1, wherein said vacuum servo amplifier comprises a dichotomous chamber separated into a vacuum section and an atmospheric section by a diaphragm, said sections communicating with each other through a small aperture in said diaphragm. 